1. Field of the Invention
The present invention relates to a method of manufacturing a magnetic recording medium, and especially to a so-called orientation treatment to align magnetic powder in a magnetic layer along a predetermined direction.
2. Prior Art
Magnetic recording medium, such as, magnetic tape used in various magnetically recording and/or reproducing apparatus, for example, audio and/or video tape recorder is formed of a non-magnetic substrate provided with a magnetic coating layer on the surface thereof. The magnetic coating layer is formed by coating a magnetic paint containing acicular magnetic powder, and binder uniformly dispersed in organic solvent. After the magnetic paint is applied on the surface of the substrate, while the paint is still wet and the magnetic powder in the coating layer is still movable, the magnetic tape is passed through a magnetic field to align the magnetic powder along a direction of the magnetic field, and then the coating layer is dried to fix the magnetic powder. By the orientation treatment, magnetic characteristics along the predetermined direction are improved, for example, the rectangular ratio which is a ratio of residual magnetic flux density to saturation magnetic flux density is increased.
In the prior art the orientation treatment was carried out by applying D.C. magnetic field to the magnetic coating layer by a permanent magnet or D.C. electromagnet. In this method, even the strength of the magnetic field was increased intended to increase the effect of the orientation, the orientation effect was not enoughly obtained, and contrary to the expectation surface smoothness of the coating layer was apt to be deteriorated.
Several methods were proposed to improve the orientation treatment. In one method, there is proposed an orientation apparatus including a main orientation magnet of parmanent magnet or D.C. electromagnet generating a magnetic field of one direction accompanied with an electromagnet applied with alternating current to generate a supplemental magnetic field with alternating polarity periodically (A.C. magnetic field) which is superposed on the main magnetic field. In another method, the orientation was carried out by applying D.C. magnetic field along a predetermined direction and an supplemental A.C. magnetic field superposed on the main magnetic field along a direction perpendicular to the direction of the main D, C, magnetic field, or mechanical vibration applied together with the main D.C. magnetic field to improve the alignment of the magnetic powder. In still another method, in addition to the main D.C. magnetic field, A.C. magnetic field was also applied to vibrate the magnetic powder to improve the orientation. However, in all of there prior art method, the orientation was essentially achieved by the main D.C. magnetic field, and the A.C. magnetic field applied was weak just to cause vibrations of the magnetic powder to be easily moved during the orientation treatment.
In the prior art method, in which the orientation was essentially achieved by D.C. magnetic field, even employing A.C. magnetic field supplementally, substantial orientation effect appears even the strength of the orientation magnetic field was less than the coercive force Hc of the magnetic powder, though enoughly high orientation effect couldn't be obtained even applying a D.C. magnetic field stronger than the coercive force of the magnetic powder, and contrary to the expectation, surface smoothness of the magnetic layer became deteriorated as the orientation magnetic field increases, as mentioned previously. There were further such drawbacks that good orientation couldn't be achieved and high rectangular ratio couldn't be obtained, when the magnetic powder which is poor in dispersion characteristics or large in magnetic agglomeration are employed in the paint, and when the paint contains large ratio of powder.
It is considered that the reason why the enoughly high rectangular ratio couldn't be obtained in the prior art method in which the orientation was essentially carried out by D.C. magnetic field, whether or not the supplemental A.C. magnetic field were applied, is that no switching of the polarity of magnetization of the magnetic powder is concerned in the process of the orientation. Here, switching means the changing of the polarity of spontaneous magnetization of each of magnetic powder. As shown in FIG. 1 a magnetic recording medium 1 coated with a magnetic paint containing acicular magnetic powder, binder and solvent is passed through an orientation magnetic field apparatus 2 along an arrow a while the magnetic paint is still wet and the manetic powder is still movable in the paint, and applied with a D.C. magnetic field generated by the orientation magnetic field apparatus 2 during passing in the magnetic field apparatus 2 to orient the acicular magnetic powder along the direction of the magnetic field. In this case, the D.C. magnetic field received by the magnetic powder on the magnetic recording medium does not run up to the desired strength H.sub.OR for the orientation suddenly at the enterance for the orientation magnetic field apparatus, however, the magnetic field is gradually increased by an affect of the orientation magnetic field apparatus as the magnetic recording medium comes close to the entrance. The strength of the magnetic field received by the magnetic powder is shown in FIG. 2. Then even the orientation magnetic field H.sub.OR is selected stronger than the coercive force Hc of the magnetic powder, the magnetic powder receives the magnetic field not stronger than the coercive force Hc of the magnetic powder for a certain period though it is short. Under the application of such magnetic field not stronger than the coercive force Hc, the switching of the polarity of the magnetization does not occure, however, the magnetic powder itself begins to rotate by the interaction between the magnetization and the magnetic field. In this case, when the spontancous magnetization indicated by an arrow a in FIG. 3A of the magnetic powder 3 is inclined to the direction of the magnetic field, the magnetic powder is relatively easily oriented along the direction of the magnetic field with a rotation angle .phi. which is smaller than 90.degree., while as shown in FIG. 4A, when the spontaneous magnetization indicated by an arrow b is inclined oppositely to the direction of the magnetic field, the magnetic powder has to be rotated with a large angle .phi. as much as 180.degree. as shown in FIG. 4A. Then, in this case until the orientation is completed the magnetic powder must be moved largely and long time is necessary to complete the orientation. Further, even the orientation magnetic field is applied for a enoughly long time, the magnetic powder is apt to be entangled with each other since each powder has to be moved largely, and the powder are apt to be fixed on half way of the orientation as shown in FIG. 4B. The tendency is more remarkable, when the powder employed is, as mentioned previously, poor in the dispersion characteristics, or large in magnetic agglomeration. Thus, in the orientation essentially carried out by using the D.C. magnetic field, there is unreasonable movement of magnetic powder which causes entanglement of the magnetic powder, which results in the poor rectangular ratio, even when strong orientation magnetic field is employed. Further there is a drawback that the surface smoothness of the coating layer becomes deteriorated.